An intimate physical and functional relationship exists between Sertoli cells and germ cells in testes of adult rats. Sertoli cell division occurs only during perinatal development, when the size of this population is established. This period may be of major importance in insuring production of sufficient sperm, i.e., fertility in adults. To test this, the PI will study spermatogenesis in adult rats that have either abnormally few or many Sertoli cells: the point(s) in germ cell maturation dependent on Sertoli cells will be identified and the impact of Sertoli cell population size on fertility analyzed by testing the ability of Sertoli cell-deficient rats to impregnate females. Although the PI identified FSH as a major factor regulating Sertoli cell proliferation in vivo, virtually nothing is known about how it initiates DNA synthesis in these cells, or about other possible levels of control over the size of this population. The PI will probe the events set in motion by FSH that culminate in Sertoli cell division by addressing: (1) the role of intracellular Ca++ in mediating FSH-induced Sertoli cell division; (2) the involvement of a Ca++-sensitive phosphodiesterase unique to immature Sertoli cells in the proliferative response; (3) the relationship among the position of a Sertoli cell in the cell cycle, its possession of functional FSH receptors and its ability to respond to FSH; and (4) the role of estrogen formation in promoting Sertoli cell division. The PI will also explore the possibility that testicular POMC-derived peptides, e.g., b-endorphin, exert paracrine control over Sertoli cell population size, thus modifying the effect of FSH, by testing the effect of either endorphin blockade or administration on Sertoli cell proliferation. Endorphin binding sites will be localized in the neonatal testis, and studies carried out to probe the nature, at the cellular level, of the mechanism by which this peptide suppresses FSH-stimulated Sertoli cell division. Studies are proposed in vitro to allow control of the conditions under study and in vivo to permit evaluation of the physiological significance of the findings. Data emerging from this work should provide new information on the role of Sertoli cell development in establishing fertility and yield new insights into how development of these cells may be modified in vivo, thus enlarging our understanding of fertility and infertility in males.